The gross selection rule for the observation of vibrational Raman transitions is that the polarisability of the molecule should change as the molecule vibrates. Both homonuclear and heteronuclear diatomics fulfill this requirement, so both molecules are vibrationally Raman active. If we approximate the potential energy curve of a vibrating bond as…

The rotational constant of a vibrationally excited state of a diatomic molecule will be slightly smaller than that of the vibrational ground state, as the excited state will have a slightly longer bond than the ground state (due to the anharmonicity of the vibration). Consequently the moment of…

If the vibrational spectrum of a gas-phase heteronuclear diatomic molecule is obtained at high enough resolution, it is found that each line of the spectrum actually consists of a large number of closely spaced components. These components arise from the rotational transitions that accompany vibrational…

The approximation of the potential energy to a parabola cannot be correct at all extensions, as it does not permit dissociation of the bond. At high vibrational excitations (i.e. in states with high values of the quantum number ν), the parabolic approximation is particularly poor. The motion at…

The gross selection rule for vibrational transitions is that the electric dipole moment of the molecule must change in the course of the vibrational motion. e.g. homonuclear diatomics are infrared inactive – stretching of the bond does not alter the dipole moment of the molecule, it remains at zero….

A typical potential energy curve for a diatomic molecule has the following form: REqm is the internuclear separation between the atoms at equilibrium – the equilibrium bond length. At smaller separations the potential energy rises rapidly as a result of repulsion between the outermost electrons. At larger separations, the…

There is another convention in the labeling of molecular orbitals that we need to be aware of. This concerns the parity of the orbital. The parity of an orbital is its behaviour under inversion. An orbital of even parity appears the same when inverted through its centre, while…

The procedure for working out a molecular orbital of a general diatomic molecule is quite simple. We construct molecular orbitals using the available orbitals on the atoms. We then fill up the molecular orbitals, starting with the lowest in energy, until all the electrons in the species have…

Molecular orbital theory does not consider the electrons in a bond to be localised between two nuclei. Rather it considers them to occupy a molecular orbital. This orbital, the region of space in which the electron is most likely to be found, covers the whole molecule,…

The fundamental aspects of valence bond theory as outlined on the previous page are that superposition of wavefunctions leads to a low energy bonding wavefunction in which the probability of finding the electrons in the internuclear region has been substantially increased. This discussion centred on the H2 molecule, which…